1. Field of the Invention
The invention relates generally to data terminal equipment and more particularly to retrofit incorporation of modems within data terminal equipment.
2. Description of Prior Art
Data terminal equipment varies in design and application, but typically includes a keyboard, a CRT (Cathode Ray Tube) and some associated electronics including a microprocessor, a varying amount of random access memory, and either a UART (Universal Asynchronous Receiver Transmitter) or a USART (Universal Sychronous Asynchronous Receiver Transmitter). The UART (or USART) converts the form of the data between the parallel form used within the data terminal equipment, and the serial data form typically used for transmission through a RS232 port and cables connecting with other equipment. The UART (or USART) is typically packaged in a dual-in-line package (DIP) and plugged into a socket in the data terminal equipment.
When the data terminal equipment must communicate with remote equipment, communication over telephone lines can be accomplished by use of a modem which typically converts between serial data signals and audible tone signals, with the audible tone signals transmitted over the telephone lines. If the data terminal equipment does not contain a built-in modem, as is true of much of the equipment which has been, and still is being manufactured, a separate stand-alone modem can be used for telephone transmission of data. A stand-alone modem will include a cabinet and a power supply along with the electronics which perform the conversion of the form of the data, and will require connection to a power source for operation. It will typically be connected to the data terminal equipment through an RS232 port, to the telephone line through an RJ11C telephone line connector, and to a 115 volt AC power source through a plug to a wall outlet.
Data terminal equipment may be made with built-in expansion capability commonly referred to as expansion slots, through which accessories and peripheral equipment may be connected. Modems are available which plug directly into expansion slots, utilizing the power supply and the cabinet of the data terminal. These modems typically cost less than stand-alone modems because they avoid duplication of functions available in the data terminal. This approach is also more reliable than stand-alone modems due to the reduction of external cables. Expansion slots are common in personal computers, but many data terminals are built without including expansion slots in order to hold down costs and allow for competitive pricing. The number of expansion slots is limited by economic considerations as to any specific design of data terminal equipment. There has been a trend toward including more features as integral parts of personal computers so as to allow utilization of features which would otherwise have required use of an expansion slot. There has also been a trend toward developing accessory cards which provide multiple features so as to reduce the number of expansion slots used in attaining given accessory features. Expansion slots are generally considered a limited and therefore valuable resource, not to be thoughtlessly expended. The ability to add features without occupying an expansion slot is therefore beneficial.
There are also data terminals available which are specifically intended for utilization for communication by telephone line, and include modems built-in as an integral part of the electronics of the data terminal. These modems also utilize the power supply and cabinet of the data terminal, and have the advantages associated with such internalization including lower cost and improved reliability when compared with the utilization of stand-alone modems. Data terminals originally manufactured with a modem integrated into internal electronics serve a smaller segment of the market than terminals which allow, but do not require expansion to include a modem.
Data terminal equipment which does not include either a built-in modem or expansion slots is the least expensive to manufacture and is therefore the easiest equipment to market at aggressively competitive prices. There is a large quantity of such data terminal equipment both in use and currently being manufactured.